Generally, pressurized cylinders, such as hydraulic or pneumatic cylinders, are used for a variety of applications where one end of the cylinder is secured to a stationary object and the other end of the cylinder is secured to a movable object. These applications may include use in cars, trucks, trailers, buildings, and various machines for such functions as lifting, pulling or towing, or controlling movement between the objects. Cylinders in these typical applications often require limitations on the amount or type of pressure or control for movement between the objects. Various valves and the like were therefore developed to enhance these limitation requirements.
Several control valves for controlling pressure to these cylinders have been used for applications such as hydraulic brake systems for vehicles, such as seen in U.S. Pat. No. 4,932,727 by Wagner et al. entitled "Automotive Vehicle Brake System", U.S. Pat. No. 4,922,120 by Becker et al. entitled "Brake System With Slip ControI", U.S. Pat. No. 4,783,128 by Resch entitled "Brake Pressure Control Unit With Hydraulic Power Booster Including Antilocking And Propulsion Regulation", U.S. Pat. No. 4,690,465 by Takeda et al. entitled "Antiskid Hydraulic Pressure Modulator For Vehicle Hydraulic Braking System", U.S. Pat. No. 4,687,259 by Reinartz et al. entitled "Hydraulic Brake System With Slip ControI", U.S. Pat. No. 4,565,265 by Woolley entitled "Hydraulic Parking Brake", and U.S. Pat. No. 4,421,212 by Fleck entitled "Brake Control Valve Arrangement."
Various valves for these cylinders have also been developed for controlling the damping movement occurring between the stationary and moving object or to control the relative position between the two objects. Examples of these valves may be seen in U.S. Pat. No. 5,004,079 by Ivers et al. entitled "Semi-Active Damper Valve Means And Method", U.S. Pat. No. 4,815,574 by Taylor et al. entitled "Frictionless Damper", U.S. Pat. No. 4,588,053 by Foster entitled "Multiple Rate Shock Isolator Damping Valve", U.S. Pat. No. 4,551,104 by Iwashita et al. entitled "Tilt Lock Mechanism For Marine Propulsion Device", U.S. Pat. No. 4,350,212 by Hirose entitled "Step Feed Drilling Machine", U.S. Pat. No. 4,334,624 by Detmold entitled "Draft Gear Locking Device", U.S. Pat. No. 4,266,639 by Schloth entitled "Quick Response Hydraulic Shock Suppressor For Piping Systems", U.S. Pat. No. 4,200,031 by Skover, Jr. entitled "Locking Cylinders", U.S. Pat. No. 3,972,557 by Hudston et al. "Hydraulic Cab-Tilting Systems", U.S. Pat. No. 3,942,551 by Schuller et al. entitled "Hydraulic Damping Means For Hinged Check Valve", U.S. Pat. No. 3,875,850 by Reynolds et al. entitled "Double-Acting Lift Cylinder With Integral Velocity Fuses", U.S. Pat. No. 3,472,547 by London entitled "Safety Tilt System", U.S. Pat. No. 3,394,634 by Pfundt entitled "Internal Relief Valve For Hydraulically Actuated Pole Pullers", U.S. Pat. No. 3,349,671 by Hoffman entitled "Holding Valve With Thermal Relief", U.S. Pat. No. 2,887,990 by Rogers entitled "Lift Cylinder Regulator Valve", and U.S. Pat. No. 3,563,137 by Graber et al. entitled "Hydraulic Self-Leveling Control For Boom And Bucket."
Pressure relief valves have also been developed for relieving pressure applied to these cylinders by fluid or air supplied to the cylinder when the cylinder receives an excessive external pressure applied to a piston rod of the cylinder or to the retractable or extendable movement of a piston rod of the cylinder. These pressure relief valves, however, are typically mounted external to the cylinder. Examples of such valves may be seen in U.S. Pat. No. 5,044,256 by Budzich entitled "Exhaust Pressurizing Control For A Fluid System", U.S. Pat. No. 5,038,825 by Hilmer et al. entitled "Multiport Valve", U.S. Pat. No. 4,834,135 by DiBartolo entitled "Pressure Control Valve", U.S. Pat. No. 4,615,401 by Garrett entitled "Automation Hydraulic Thruster", U.S. Pat. No. 4,611,621 entitled "Pressure Control Valve And Oil Supply Device Using Said Valve", U.S. Pat. No. 4,591,314 by Weber entitled "Hydraulic Power Supply System Utilizing A Solid Propellant Gas Generator", U.S. Pat. No. 4,433,615 by Vick entitled "Blocking and Thermal Relief Valve", U.S. Pat. No. 4,114,516 by Johnson entitled " Anti-Cavitation And Pressure Modulating Relief Valve For Controlling Hydraulic Cylinders", and U.S. Pat. No. 3,887,160 by Cusveller entitled "Fluid Operated Actuator For Movable Members." Some of these valves have included pressure relief for both retractable and extendable movement as seen in U.S. Pat. No. 4,194,436 by Imada entitled "Speedup Device For Reciprocating Cylinders" and the book "Industrial Hydraulics", pp. 274-75 (3d ed. 1979) by Pippenger and Hicks.
Other pilot operated valves have been developed for these pressurized cylinders where pressure has been relieved by pilot operation of a third party or some other type of mechanical activation external to the cylinder housing. Examples of these pilot operated valves may be seen in U.S. Pat. No. 5,065,664 by Ohta et al. entitled "Control Circuit For A Cylinder Allowing Flow Between An Upper And A Lower Chamber", U.S. Pat. No. 4,718,330 by Mitton entitled "Hydraulic Cylinder", U.S. Pat. No. 4,667,570 by Jensen, Jr. et al. entitled "Integral Hydraulic Blocking And Relief Valve", U.S Pat. No. 4,461,449 by Turner entitled "Integral Hydraulic Blocking and Relief Valve", and U.S. Pat. No. 3,074,384 by Pilch entitled "Pilot-Operated Ball Check Valve In Cylinder Head."
These prior cylinders and valves, however, have multiple problems associated therewith. Some of these cylinders and valves are used for controlling the damping motion occurring during movement between the objects or for controlling the speed of the movement, such as when lifting a trailer bed off of a truck. Others have been used for specific applications such as hydraulic brake systems or are pilot operated wherein a third party is required to relieve the pressure or a separate pilot operated mechanical device is used.
Still others have pressure relief valves external to the cylinders which make the cylinders bulky, awkward, and difficult to use in compact spaces. Because of the many fluid lines required when an external pressure relief valve is connected to these cylinders, the pressurized system is more susceptible to mechanical failure due to the mounting of the cylinder, the rough conditions of usage, and the limited space where these cylinders may be installed.
Therefore, there is a continual need for a pressure-actuated cylinder having an internal pressure relief valve built into the cylinder which directly relieves pressure applied to the cylinder from either retraction or extension of the cylinder piston.